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The Third Ventricle and

anterior, fornix, commissure, lateral, aqueduct and roof

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THE THIRD VENTRICLE AND The inter-brain (diencephalon) is median in position (Figs. 33, 34, 35, 37 and 50). It is situated beneath the fornix and the layer of epithelium extending from the border of the fornix to the stria terminalis. The chorioid tela of the third ventricle only intervenes between them. Laterally, it is bounded by the superior lamine of the internal capsules. The ventricle of the inter-brain is the third in number. The third ventricle, there fore, is located in the median plane; and is at a lower level than the ventricles of the hemispheres. It is made up of two parts in the adult, which embryologically are distinct: the posterior part, that lying between the thalami, is the proper ventricle of the diencephalon; the small anterior part, called the aula, is the median ventricle of the telencephalon, with which the lateral ventricles communicate through the interventricular foramina. Posteriorly, the third ventricle is joined to the fourth ventricle by the cerebral aqueduct. It is a narrow, vertical cleft about 2.5 cm. (r in.) in length from before backward and 6 mm. (0.25 in.) broad at its widest part. It separates the thalami and extends almost to the inferior surface of the cerebrum. The roof (Figs. 35, 44, 48, 5o and 54) follows the curve of the fornix and arches from the posterior commissure forward to the anterior commissure. There is a little recess above the anterior commissure and between the columnce of the fornix, bounded in front by the inferior angle of the septum pel lucidum, called the recessus triangularis, in which the roof and anterior wall meet. The anterior wall extends from the gular recess down to the optic recess, at the angle between the lamina terminalis and the optic chiasma. This angle is so named because on either side of it there is a lateral extension of the third ventricle between the lamina terminalis and the columna of the fornix, which is located in the root of the em bryonic optic vesicle. The floor (Fig. 34) describes two arches, convex toward the ventricle. The first arch, very convex and

short, stretches between the optic recess and the infundibulum, in which the floor reaches its lowest point. The distance from the infundibulum to the anterior orifice of the cerebral aqueduct is spanned by the second arch. It is long and flat. Its posterior extremity is but 1.7 mm. (0.07 in.) below the pos terior commissure; the anterior orifice of the cerebral aqueduct separates them. The ventricle is thus contracted behind to the size of the cerebral aqueduct with which it is continuous. The lateral walls (Figs. 34 and 37) are close together throughout. At one point near the middle they come together and are joined by the massa intermedia (middle commissure). Antero superiorly, the lateral wall is perforated by the interventricular foramen (of Monro). That foramen constitutes the slight separa tion between the front of the thalamus and the columna of the fornix. It opens into the lateral ventricle at the junction of the anterior horn with the central part. The ependyma which lines the third ventricle is continuous through the interven tricular foramen with the lining of the lateral ventricle. But one layer of the ependyma is present in the roof of the ventricle; that is the epithelial layer. The third ventricle, like all true ventricles, is occupied by cerebrospinal fluid.

The following are the boundaries of the third ventricle: Roof— Posterior commissure and commissura habenularum, Roof epithelium and pineal body, Chorioid tela and plexuses, Fornix and commissura hippocampi.

Anterior wall— Epithelium, covering Column of fornix, anterior commissure, and Lamina terminalis.

Floor— Optic chiasma, Tuber cinereum and infundibulum, Corpora mammillaria, Posterior perforated substance (of mid-brain), Tegmenta (of mid-brain).

Posteriorly— Ventricle is continuous with cerebral aqueduct.

Lateral wall— Thalamus and reflected hypothalamic substance, Columna of the fornix, and Foramen interventricular between them.

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